This invention relates to optical projection apparatus for an image transfer type electrophotographic copier, using an image transmitter formed by a bundle of optical fibers having graded refractive indexes.
An image transmitter formed by a bundle of optical fibers having graded refractive indexes referred to herein is an element consisting of a number of optical fibers 4 arranged in intimate contact with one another, and a material such as a resin which encloses the plurality of optical fibers. The refractive index is graded from the longitudinal axis of the optical fiber towards the peripheral portion thereof, as shown in FIG. 1. As shown in FIG. 2, if the length l of the plurality of optical fibers is properly selected, then there is obtained an erect image at a ratio of 1:1 at a distance l' from the bottom end of the optical fibers with the original document at a distance l' from the top end of the optical fibers.
The image transmitter formed by a bundle of optical fibers 2 having graded refractive indexes is advantageously small as compared with a optical lens system of the prior art, and also does not decrease the quantity of peripheral light or the resolving-power of the optical system. Various attempts have recently been made for using an image transmitter formed by a bundle of optical fibers having graded refractive indexes in a copying machine. For example, a device as shown in FIG. 2 has been proposed, in which an original document 6, the image transmitter formed by a bundle of optical fibers 2 having graded refractive indexes and photosensitive drum 8 are disposed in vertically aligned relation to one another. That arrangement, however, requires that original document 6 and photosensitive drum 8 be spaced apart a certain distance from each other, with the result that the size of the copying machine is not reduced, because of the aforesaid intervening distance requirements.
The present invention achieves a compact copying machine size, and particularly a reduction in the height of the copying machine, by providing prism-shaped reflecting members at both ends of an image transmitter formed by a bundle of optical fibers having graded refractive indexes, so as to refract the optical path. For a better understanding of the present invention, there is shown in FIG. 3 a prior art device using an image transmitter formed by a bundle of optical fibers having graded refractive indexes as well as a prism, which is disclosed in Japanese patent application No. 48-139287 published on July 24, 1975. In this prior art device, the image transmitter formed by a bundle of optical fibers 2 having graded refractive indexes is used as an optical system for an electrofax type copying machine. Since photosensitive sheet 10 coated with a photosensitive material such as ZnO is used in an electrofax type copying machine, the optical image of original document 6 must be projected onto photosensitive sheet 10 in a manner to be developed as a real image. Accordingly, in case the image transmitter formed by a bundle of optical fibers having graded refractive indexes, which forms an erect image, is used in the electrofax type copying machine, then the optical image need be inverted once by a prism or mirror. To this end, there is provided in the device of FIG. 3 prism 12 or a mirror at the end of the image transmitter formed by a bundle of optical fibers 2 having graded refractive indexes. The image thus inverted is formed as an electrostatic latent image, conforming to the image of the original document, on photosensitive sheet 10 charged with electrostatic charges beforehand, and then subjected to development in a known developing device.
In this device, if prism 12 is formed so as to provide only that portion which is optically required, then there is required an increase in the length of the prism 12 compared with its volume, and such an elongated prism 12 is susceptible to deformation, thus causing image distortion. Such an elongated prism is also difficult to mold. Furthermore, the use of a bonding agent for fixing the prism to an image transmitter formed by a bundle of optical fibers having graded refractive indexes involves a risk of misalignment between the prism and the image transmitter due to heat incurred during the operation of the photocopier.